Polymeric films such as PET film are commonly used to transfer metal to paperboard and other substrates for use in packaging, greeting cards, and similar product applications where it is desirable to give the product a metallic appearance. The technique of transferring metal from film to another substrate is used where it is not practical to metalize such substrates directly.
One typical metal transfer technique uses a metalized conventional polymeric film and bonds the complete metalized film structure to the substrate. Typical methods for depositing metal onto the polymeric film include vapor and sputter metallization processes. Such deposition techniques typically create strong bonds between the metal and the film. Because metal does not readily separate from the polymeric layer of the film, in the transfer procedure the paperboard substrate becomes permanently bonded to the metalized polymeric film. This can create excessive non-recoverable scrap since the resulting laminate, for example generated by post-consumer packaging disposal, can not be easily recycled.
Another technique that is well known and practiced in industry is to create a carrier film that can be used to transfer a metal layer to paperboard and other substrate materials. The carrier film is created by coating a layer of release material on the base polymeric film in a secondary, separate process from producing the base polymeric film. Subsequently, and in an entirely separate third process, a metal layer is deposited onto the release layer for example by via vapor or sputter metallization. The individual steps may be carried out at different locations by different converters. A disadvantage is that an extra processing step is required, in particular the step of applying a release layer. A need exists for biaxially oriented polyester films that can transfer metal from a layer deposited onto the carrier film surface to another substrate without the need for costly coatings between the metal layer and the carrier film surface.
The primary function of the release layer is to provide appropriate adhesion between the polymeric film surface and the metal layer. The adhesion of the metal to the film surface should be strong enough to endure handling in manufacture, packaging, shipping, etc. prior to metal transfer. However, adhesion should be sufficiently weak that the metal layer cleanly separates from the carrier film surface when contacted with the substrate.
Them is a need for a method of metal transfer from a polymeric film to a substrate in which a metal layer can be applied directly onto the polymeric film and in which the metal layer readily releases and separates from the film. It is desirable to have a carrier film that does not have an added release layer on the surface of the polymeric film. It is further desired to have a carrier film for metal transfer in which the base layer is a single polymeric composition that can be recycled after the metal has been transferred. Still further it is desired to have a polyester-based carrier film and transfer film that is free of a non-polyester release layer.
Basically, metal is transferred by contacting an adhesive-coated substrate with the metal layer side of the polymeric carrier film composite and then peeling the polymer away to leave the metal on the substrate. The ability to make the transferred metal on the substrate highly glossy is very desirable. However, generating a high gloss appearance after removing the polymeric film is made problematic by conventional film “anti-blocking” technology. Polymeric film tends to stick to itself and other surfaces, sometimes referred to as “blocking,” during various film handling steps of the metal transfer operations. Typically, blocking is reduced by uniformly dispersing fine inorganic particles in the polymer of the carrier film. The particles give the film a rough surface texture that renders the film less susceptible to blocking. Because the metal layer profile conforms to the carrier film surface, after removal of the polymer the transferred metal exhibits a rough surface texture. Unfortunately, surface roughness reduces gloss such that the transferred metal has a low gloss or even matte finished appearance. A very smooth transferred metal surface, for example as might be formed without particles embedded in the polymer, should have high gloss appearance but is impractical to achieve because friction between adjacent polymer film layers makes film handling unmanageable. Thus there is an important need to have a polymeric film free of a separate release layer that can transfer metal to a substrate with superior gloss.